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Microwave-Assisted Green Synthesis

Climate change, resource shortages, and budget cuts for research and development are forcing scientists to rethink traditional methods. While developing new approaches focused on environmental friendliness, chemists introduced the term “green chemistry”[1] to capture the idea of environmentally friendly design of chemical processes. The term green synthesis is now widely used to indicate chemical reactions that are performed employing environmentally benign and sustainable processes/materials. In order to structure approaches to this sustainable idea, 12 principles have been defined, each one bringing chemists closer to green synthesis. These principles are:[2]

  1. Waste prevention
  2. Maximization of atom economy
  3. Less hazardous syntheses
  4. Design of safer chemicals
  5. Usage of safer solvents and auxiliaries
  6. Energy efficiency
  7. Usage of renewable feedstocks
  8. Reduction of chemical derivatives
  9. Catalysts
  10. Design for degradation
  11. Real-time analysis for pollution prevention
  12. Safe chemistry for accident prevention

Since modern sealed-vessel microwave systems support many of the above-mentioned principles, they are well-suited for microwave-assisted green synthesis. 

Waste prevention in microwave-assisted green synthesis

Microwave-assisted synthesis only makes sense under sealed vessel conditions.[3] Consequently, when sealing the vessels, the waste of water for reflux heating (or actually cooling) is reduced to zero since reflux condensers are of no use. Dedicated microwave reactors cool the reaction mixtures either by compressed air or by fan, which makes them perfectly suited for microwave-assisted green synthesis.

Maximization of atom economy in microwave-assisted green synthesis

The most important beneficial effect of microwave-assisted synthesis compared to conventionally heated reactions is that the reaction times are shortened and the yields are improved. There are thousands of examples that show this beneficial effect.[4] Therefore, regarding this special requirement, almost any microwave-assisted reaction is a step towards green synthesis.

Usage of safer solvents and auxiliaries in microwave-assisted green synthesis

There used to be a trend towards green synthesis to perform more and more reactions in water instead of organic solvents, if possible.[5-7] Now, though, microwave chemists also focus on “neat” reactions – reactions that are done without any solvent.[8-10]

Another interesting improvement towards microwave-assisted green synthesis was shown from Glasnov et al. They demonstrated that microwave synthesis – far above the boiling point of the used solvent – can help to avoid acid catalysts, which are required under conventional reflux heated conditions.[11]

Apparently, simple heating to 160 °C lets the reaction work without acid catalyst. At room temperature, however, sulfuric acid is required to give similar results. This is a very effective demonstration of microwave-assisted green chemistry.

Energy efficiency in microwave-assisted green synthesis

Some time ago, the Kappe research group compared the energy consumption of conventional hot plates with microwave reactors.[12] It showed with different reaction types (Diels-Alder, hydrolysis, Suzuki coupling, and cyclocondensation) that, compared to conventionally heated experiments, microwave-assisted synthesis consumes far less energy. These findings are based on the fact that under microwave conditions, reactions simply proceed faster until full conversion is reached. The results are very impressive and show that microwave-assisted synthesis indeed makes chemistry “greener”.

Catalysis in microwave-assisted green synthesis

Unique accessory for microwave reactors aid in performing efficient catalyst screening (see picture below). With special microwave equipment, up to 96 reactions can be performed in parallel. This way, you can find the optimum catalyst, reaction conditions, or reagent equivalents for your reaction, saving you time and energy for your next 95 further runs.[13-15]

Besides parallel synthesis, successful catalyst screening was also done in an automated microwave-assisted system. A 24-position autosampler, placed on top of the microwave synthesis reactor Monowave 400, automatically handled 24 reaction mixtures with individual catalysts to find the optimum catalyst for the present reactions.[16-17]

Real-time analysis for pollution prevention in microwave-assisted green synthesis

An interesting step towards green synthesis is progress analysis of chemical synthesis in real time. This means that the progress of the reaction can be followed with some kind of detection, and it becomes immediately clear when a reaction is finished. This saves time and energy and further chemicals. 

Ideally, the detection is done spectroscopically. Monowave 400R – so far, the only microwave reactor with an integrated spectrometer – provides microwave-assisted green synthesis not only due to the benefits of sealed-vessel chemistry, but due to its integrated Raman spectrometer it can give you immediate feedback about the course of the reaction.[18]

Besides spectroscopic real-time analysis, microwave reactors feature other accessories for real-time analysis to support approaches to microwave-assisted green synthesis: Even the use of an in-built camera can give you important insights into the progress of the reaction, as shown in the picture.

Shown is the hydrazine mediated formation of Fe3O4 nanoparticles from nitrobenzene and Fe(acac)3 in methanol.[19] Once the particles are formed, the yellowish color of the mixture disappears while the Fe3O4 nanomaterials precipitate. 

Another example was shown by Tang et al. who did microwave-assisted green synthesis by following the formation of fluorescent amine-capped carbon dots with the integrated camera.[20]

Safe chemistry for accident prevention in microwave-assisted green synthesis

As mentioned above, there is an approach towards microwave-assisted green chemistry to perform reactions neat – without any solvent.[6] This reduces the risk of explosions due to the absence of explosive, low-boiling solvents.

Besides this, modern dedicated microwave-synthesis reactors adhere to the highest safety standards. Because they can reach temperatures up to 300 °C and pressures up to 80 bar, the safety standards are very high in order to keep everything under safe control. In line with safety regulations, the instruments are designed in order to minimize the risk for potential hazards to provide maximum safety for chemists and the environment. 


[1] Anastas, P. T., Warner, J. C. (2000). Green Chemistry: Theory and Practice. Oxford University Press
[2] American Chemical Society (2023). https://www.acs.org/greenchemistry/principles/12-principles-of-green-chemistry.html 
[3] Refer to Chapter 3.3.2. of Kremsner, J. M., Stadler, A. (2018). A Chemist’s Guide to Microwave Synthesis, 3rd Edition. Anton Paar Publishing, Graz, https://www.anton-paar.com/us-en/synthesis-guide/
[4] Kappe, C. O., Stadler, A., Dallinger, D. (2012). Microwaves in Organic and Medicinal Chemistry, 2nd Edition. Wiley-VCH, Weinheim
[5] Baumgartner, B. et al. (2016). Macromol. Chem. Phys. 217, 485 
[6] Ganji, P. (2020). Biomass Conv. Bioref. 10, 823
[7] Amaya-García, F. et al. (2021). ChemSusChem 14, 1853
[8] López-Lira, C. et al. (2021). Bioorg. Chem. 111, 104823
[9] Santos, A. F. M. et al. (2022). Liq. Cryst. 49, 1809
[10] Bijalwan, K. et al. (2022). ChemNanoMat 8, e202200044
[11] Glasnov, T. N. et al. (2009). ChemMedChem 4, 1816
[12] Razzaq, T., Kappe, C. O. (2008). ChemSusChem 1, 123
[13] Jacobsen, J. et al. (2019). Dalton Trans. 48, 8433
[14] Dreischarf, A. C.  et al. (2017). Inorg. Chem. 56, 2270
[15] Damm, M., Kappe, C. O. (2012). Mol. Divers. 16, 5
[16] Karmakar et al. (2019). New J. Chem. 43, 9843
[17] Frija, L. M. T. et al. (2020). J. Mol. Struct. 1222, 128831
[18] Hebert, O. et al. (2022). Synthesis 53, 5215
[19] Cantillo, D. et al. (2013). J. Org. Chem. 78, 4530
[20] Tang, H. et al. (2022). Colloid Surface A, 651, 129564